Optical recording and data storage material having different optical characteristics are known. For example, U.S. Pat. No. 4,269,917 to J. Drexler and E. Bouldin, assigned to the assignee of the present invention, for "Data Storage Medium Having Reflective Particulate Silver Layer," describes a reflective laser recording and data storage medium for direct reading after writing. It is formed from the conversion of a photosensitive silver halide emulsion into a reflective, stable silver-gelatin coated substrate in which the silver-gelatin coating is easily pitted by impingement of a laser beam.
The chemical and physical conversion of the raw silver-halide emulsion materialinto the laser recording material uses the following exemplary steps. First, a non-saturating actinic radiation exposure is used to activate silver-halide in the form of a fine grain emulsion on a substrate, such as a photoplate, thereby defining areas for data recording. Alternatively, data areas on the surface of the medium may be chemically fogged in a water or alcohol base solution to create a very thin layer of silver precipitating nuclei on the surface. A single step, negative silver diffusion transfer process is used to dissolve the unexposed and undeveloped silver halide, forming silver ion complexes. These complexes are transported by diffusion transfer to the sites of the silver precipitating nuclei where reflective silver particles are formed. The resulting reflective coating has a high concentration of nonfilamentary silver particles at the surface of a low melting temperature colloid matrix. For a typical laser wavelength used in laser data retrieval systems the reflective surface layer and underlayer have a composite reflectivity ranging between 15% and 65%.
Laser writing on this recording material is accomplished by making holes or pits or clear spots, all known as "pits" hereafter, in the reflective surface. A laser beam or focussed light beam is used for reading recorded data. The beam impinges on the recorded pits with greatly reduced specular reflection due to scattering and absorption by the pitted underlayer. The reductions in reflectivity are measured by a detector and converted to electrical impulses corresponding to data.
One of the advantages of this medium is that it also can be photographically prerecorded to make clear or partially opaque spots in a reflective field. Such partially opaque spots are spots which are opaque at some wavelengths and transmissive at other wavelengths. In this sense opaque spots are equivalent to clear spots and are grouped into the classification of "pits" mentioned above. In the first non-saturating exposure step a pattern can be formed by exposure through a mask or scanning light source, which after processing, yields two different surface reflectivities. This pattern resides both in the reflective layer and in the underlayer, below the reflective surface layer, or in laterally adjacent areas.
Another form of optical media having different spectral qualities is disclosed in U.S. Pat. No. 4,304,848. Replication techniques are disclosed for prerecording opaque data spots in a reflective field. An unexposed silver halide emulsion is exposed to actinic radiation through a master disc having clear data spots in an opaque field. The exposed area of the emulsion corresponding to the master's clear data spots is then developed black. Exposure of the surface of the emulsion to chemical fogging and development in a monobath then converts the remaining unexposed silver halide to a reflective silver background. Opaque data spots appear against a reflective field on the prerecorded copy. By use of a master disc having opaque data spots in a clear background it is possible to obtain reflective data spots in an opaque field by the same procedure.
One of the problems which occurs in reading differences in reflected light from the optical media is that it is difficult to distinguish between a change of reflectivity due to the presence of a pit and that due to the presence of dirt particles or material defects which might affect light scattering or absorption. U.S. Pat. No. 3,919,447 to C. C. Kilmer, Jr. et al for "Spectral Diffrential Coded Card" teaches a data card made of two films. The first film has a transmission bandpass in the infrared. The second film is opaque at a single frequency in the first film bandpass. The differences in spectral response allows a beam operating at the single frequency to read data.
U.S. Pat. No. 4,145,758 to J. Drexler and C. Betz, assigned to the assignee of the present invention for "Error Checking Method and Apparatus for Digital Data in Optical Recording Systems" describes a data reading system wherein digital data is written onto a transmissive medium, such as a photoplate, by a modulated laser whose beam is detected by a first photodetector means which measures laser output directed toward the recording medium. A second photodetector means measures light scattering from the medium, while a third photodetector detects and measures light transmitted through the recording layer of the medium surface to confirm recording of the data. Amounts of transmitted light or scattered light from the medium during the recording process are correlated to the laser output into expected values of light for detecting errors in recording immediately after the time of recording. This error detection system is intended for light transmissive media and would not be used in reading reflective media. The defects are detectable by the apparatus before laser recordings themselves are detectable.
An object of the present invention is to read photographically prerecorded data pits in the form of partially opaque or clear spots in a reflective field, or data pits in the form of reflective data spots in a partially opaque or clear field, with a low error rate, reducing the effects of dirt and material defects in the recording medium.